1. Field of the Invention
The present invention relates to a primary radiator for circularly polarized wave, in particular, to the provision of a primary radiator for circularly polarized wave which makes it possible to realize wide-band uniformity of axial ratio as well as to obtain a satisfactory directivity for circularly polarized wave, without expressly increasing the size of the device.
2. Description of the Prior Art
Referring to FIG. 1, a simplified cross-sectional view of a prior art primary radiator for circularly polarized wave is shown with reference numeral 10. In the figure, the section between A--A' and B--B' is a conical horn antenna 12, and the section between B--B' and C--C' which joins to the above is a circularly polarized wave generator 14. The circularly polarized wave generator 14 is for converting a linearly polarized wave (electromagnetic wave) to a circularly polarized wave. As is well known, conversion of a linearly polarized wave E to a circularly polarized wave is accomplished by decomposing E into mutually orthogonal components E.sub.1 and E.sub.2 and delaying (or advancing) the orthogonal incident electric field E.sub.1 by 90.degree. with respect to the incident electric field E.sub.2, as shown in FIG. 1. To achieve this, a pair of conductor pieces 18 and 18' are provided on the inner side of a circular waveguide 16.
According to the prior art, a primary radiator for circularly polarized wave has been developed with horn antenna 12 and circularly polarized wave generatror 14 as mutually independent, and it has been put to practical use by coupling these parts to each other. However, when the frequency characteristics of the axial ratio which represent the quality of the circularly polarized wave is attempted to be valid uniformly over a wide range of frequency, the prior art radiator gives rise to various kinds of difficulties as will be described below.
As an example of an antenna in which wide-band uniformity of axial ratio is required, one may mention the antenna for receiving satellite broadcast in the 12 GHz band. In this instance, Japan is assigned a band of 300 MHz, while the United States is assigned a band of 500 MHz, by the World Administrative Radio Conference (WARC-BS).
In the prior art circularly polarized wave generator 14, it becomes necessary to reduce the thickness D of the conductor pieces 18 and 18' in order to assure the wide-band uniformity of axial ratio. In that case, however, there is a disadvantage that the axis of the circular waveguide has to be made long. The reason for this is as follows. The result of study of the frequency characteristics of the phase difference, when the thickness D of the conductor pieces 18 and 18' in the circular waveguide 16 of radius R=12.0 mm is varied from 3.6 mm to 2.4 mm and 1.2 mm, is as shown in FIG. 2. It should be noted in this case that a perfect circularly polarized wave is designed to be obtained for the frequency of 12.45 GHz with a phase difference of 90.degree.. As may be seen from FIG. 2, uniformity of axial ratio can be accomplished through decrease in the valve of D, with a reduction in the deviation of the phase difference from 90.degree. over a wide range of frequency. In this case, however, the length of the conductor pieces along the axis of the circular waveguide is found to increase gradually from 36.7 mm, 78.0 mm to 297.5 mm. In other words, with the prior art system, the total length of the primary radiator for circularly polarized wave is increased necessarily, and the system is rendered large in size, when wide-band uniformity of the axial ratio characteristic for circularly polarized wave is attempted.
On the other hand, when the phase difference between the orthogonal components of the electric field was examined for the values of radius R from 8.12 mm and 10.1 mm to 12.0 mm, by fixing the ratio D/R of the thickness D of the conductor pieces to the radius R of the circular waveguide at a constant value, for instance, D/R=0.1, a result as shown in FIG. 3 was found to exist. Here, the center frequency is chosen at 12.45 GHz at which a phase difference of 90.degree. is set to be achieved to realize a perfect circularly polarized wave there. As may be clear from the figure, the axial ratio characteristic approaches flat with decreasing deviation from 90.degree. as the radius R is increased. That is, it will be seen that the axial ratio characterictic can be made uniform over a wide range of frequency. Even in this case, however, reduction in size and weight cannot be accomplished since wide band uniformity is realizable only by increasing the radius R of the circular waveguide.
Further, as another example of the prior art, there is known a primary radiator for circularly polarized wave which has a large number of pairs of vertical plates provided at the opposite corners on the inside of a rectangular horn antenna, for converting a linearly polarized wave to a circularly polarized wave. Generally speaking, in the case when the waveguide is constructed with uniform cross section and straight tube axis, and when there is no obstacle on the tube wall, each mode of the multiple modes in the waveguide propagates independently without mutual interference. However, if obstacles such as multiple pairs of vertical plates are installed in the interior of the waveguide, then the mode independence can no longer be maintained and mode coupling will be generated. For instance, when a large number of metallic plates or the like are placed inside the waveguide, the boundary conditions at these points become discontinuous and the electromagnetic wave undergoes a large scattering there. Consequently, the mode of the electromagnetic wave in the waveguide becomes a disurbed one that includes many higher order modes other than the fundamental mode at the discontinuity points, necessarily deteriorating the characteristics of the circularly polarized wave. Therefore, a radiator with a plurality of vertical plates, as mentioned in the above, has a disadvantage in that satisfactory directivity for circularly polarized wave cannot be obtained due to inclusion of many higher order modes.